Compasses for vehicles such as automobiles, typically, are after-market items of a relatively basic mechanical design. Such compasses, typically, attach to the dashboard of the vehicle and are of inexpensive construction including a compass card suspended by a pin bearing and mounted within a housing. Usually, they will include some elemental form of adjustment or compensation mechanism such as movable magnets for adjusting the compass to north and south, depending on the geographical area of use of the vehicle.
The theory of operation of flux-gate magnetic sensors is well known and one application to the automotive environment is described in an article entitled "Magnetic Field Sensor and its Application to Automobiles" by Hisatsugu Itoh published on Feb. 25-29, 1980 by the Society of Automotive Engineers, Inc. as SAE Technical Paper No. 800123. In the commercial embodiment of this system, the bandpass filters incorporated to detect the second harmonic of the excitation input frequencies for the sensor must be critically tuned such that the quadrature information can be accurately detected and subsequently employed to drive a display of the vehicle heading. Not only must the system, however, be critically tuned, the stability of the bandpass filter and its power supply must be designed to be substantially drift free in order to provide accurate information to the phase detectors such that accurate direction display results.